(a) Field of the Invention
This invention relates to earth drilling machines such as blast hole and water well drills.
(B) Description of the Prior Art
Earth drilling machines commonly deliver drilling force to the drill string through a motor that drives a drilling head, mounting the drill string and an associated rotary drive, downwardly along a vertical mast. If the drill encounters hard rock that arrests penetration the upward reaction force upon the machine can reach the full, available downward force that may be applied to the drilling head. For some machines this upward reaction force can exceed the weight of the machine, and consequently the machine will rise upward with a resultant decrease in machine stability. The loss of stability imposes a danger to both the machine and its operator.
To alleviate the problem of an unstable condition, the maximum downward drilling force can be limited, but this results in a loss of efficiency in the drilling apparatus. An approach to maintaining a near maximum drilling force is shown in U.S. Pat. No. 3,734,202 issued May 22, 1973 for an "Automatic Feed Control System". There, a physical rise, or lift of the drilling machine is sensed by a motion responsive switch, and pressurized fluid being delivered to a downfeed motor is then partially, or fully shunted from the motor until the machine settles back down on its supporting jacks. Thus, an actual upward motion of the machine that could decrease stability must occur as a condition precedent to limiting the downfeed force. It would be superior to automatically limit the increase in downfeed drilling force before any rise of the machine occurs, and the present invention is directed to this objective.